A polarizing effect can be generated in glasses containing silver, copper, or copper-cadmium halide crystals. These crystals can be precipitated in aluminosilicate glasses having compositions containing suitable amounts of an indicated metal and a halogen other than fluorine.
The polarizing effect is generated in these crystal-containing glasses by stretching the glass, and then exposing its surface to a reducing atmosphere. The glass is placed under stress at a temperature above the glass annealing temperature. This elongates the glass, and thereby elongates and orients the crystals. The elongated article is then exposed to a reducing atmosphere at a temperature above 250.degree. C., but not over 25.degree. C. above the glass annealing point. This develops a surface layer in which at least a portion of the halide crystals are reduced to elemental silver or copper (hereafter "metal").
The production of a polarizing glass, then, involves, broadly, these four steps:
1. Melting a glass batch containing a source of silver, copper, or copper-cadmium and a halogen other than fluorine, and forming a body from the melt,
2. Heat treating the glass body at a temperature above the glass strain point to generate halide crystals having a size in the range of 200-5000 .ANG.,
3. Stressing the crystal-containing glass body at a temperature above the glass annealing point to elongate the body and thereby elongate and orient the crystals, and
4. Exposing the elongated body to a reducing atmosphere at a temperature above 250.degree. C. to develop a reduced surface layer on the body that contains metal particles with an aspect ratio of at least 2:1.
The growth of halide particles cannot occur at temperatures below the strain point of the glass because the viscosity of the glass is too high. Higher temperatures, above the annealing point, are preferred for crystal precipitation. Where physical support is provided for the glass body, temperatures up to 50.degree. C. above the softening point of the glass can be employed.
The production process is described in detail in U.S. Pat. No. 4,479,819 (Borrelli et al.). There it is pointed out that the halide crystals should have a diameter of at least about 200 .ANG. in order to assume, upon elongation, an aspect ratio of at least 5:1. When reduction to elemental metal particles occurs, the particles having an aspect ratio of at least 5:1 will display an aspect ratio greater than 2:1. This places the long wavelength peak at least near the edge of the infrared region of the radiation spectrum, while avoiding serious breakage problems during the subsequent elongation step. At the other extreme, if the diameter of the initial halide particles exceeds about 5000 .ANG., significant haze develops in the glass. This is accompanied by a decreased dichroic ratio resulting from radiation scattering.
The dichroic ratio is a measure of the polarizing capability of a glass. It is defined as the ratio existing between the absorption of radiation parallel to the direction of elongation and the absorption of radiation perpendicular to the direction of elongation. To attain an adequate ratio, the aspect ratio of the elongated halide crystals must be at least 5:1 so that the reduced metal particles have an aspect ratio of at least 2:1.
Crystals having a small diameter demand very high elongation stresses to develop a necessary aspect ratio. Also, the likelihood of glass body breakage during a stretching-type elongation process is directly proportional to the surface area of the body under stress. These are very practical limitations on the level of stress that can be applied to a glass sheet, or other body of significant mass. In general, a stress level of about five thousand psi has been deemed to be a practical limit.
The literature indicates that firing of the elongated body in a reducing atmosphere should be undertaken at temperatures above 250.degree. C., but no higher than 25.degree. C. above the annealing point of the glass. A reduction temperature as high as is compatible with the tendency for crystals to respheriodize is desirable. The time required decreases dramatically with increase in temperature. In particular, there is an abrupt change in the time required to achieve complete reduction above 400.degree. C., that is, above the melting temperature of the metal halide phase. It is thought, although not clearly proven, that the metal from the halide phase grows considerably faster when the halide phase is molten. This experimental fact means that, to carry out the reduction treatment in a practical time interval, requires a temperature above 400.degree. C., preferably above 415.degree. C. Looking at the phenomenon in another way, in order to produce, in a reasonable time, a depth of reduced layer necessary for a high contrast, the reduction treatment must be carried out at a high temperature.
One of the key measures of the effectiveness of a polarizing glass body is its contrast ratio, or, as referred to in the art, contrast. Contrast comprises the ratio of the amount of radiation transmitted with its plane of polarization perpendicular to the elongation axis to the amount of radiation transmitted with its plane of polarization parallel to the elongation axis. In general, the greater the contrast, the more useful, and valuable, the polarizing body.
Another important feature of a polarizing body is the bandwidth over which the body is effective. This property takes into consideration not only the degree of contrast, but the portion of the spectrum within which the contrast is sufficiently high to be useful. A contrast ratio of 100,000 has been taken as a point of reference for comparison purposes. Clearly, the lower the reference contrast, the broader the corresponding bandwidth. We have chosen 100,000 (50 db) because it represents a common high performance value specified for polarizer applications.
The peak contrast wavelength is determined by the aspect ratio of the elongated particle. The aspect ratio increases with the degree of stress applied to stretch the glass, and thereby the crystals. The wavelength at which the peak contrast occurs increases with the aspect ratio. Most applications in the infra-red require a peak in the wavelength range of 1300-1550 nm. However, other applications require contrast peaks outside this range, for example, as low as 600 nm.
Heretofore, it has been necessary to produce polarizing glass articles on an individual basis. Thus, it was necessary to design a separate set of processing conditions tailored to provide the peak contrast for each application wavelength. Then care had to be taken to control the process quite rigidly. The particle elongation is controlled by controlling the elongating stress applied.
The maximum bandwidth available heretofore has been about 300 nm, with a commercially practical figure being no more than 200 nm.
For example, an article might be designed having a center wavelength (CWL), that is, a contrast peak, at about 900 nm. The article would, however, have an optimum bandwidth of about 200 nm covering the range of 800-1000 run. As a result, the article would not be effective at wavelengths outside this range, e.g. 1240, 1310 and 1560 nm.
It would, of course, be highly desirable to provide a polarizing glass having a much broader bandwidth of contrast ratios above the practical use level that is now available. Ideally, this would extend from the visible into the infrared portions of the spectrum.
It is then a basic purpose of the present invention to meet this need. Another purpose is to provide a polarizing glass that is effective over a broad range of wavelengths. A further purpose is to provide a single polarizing glass article that is broadly useful in a variety of applications. A still further purpose is to provide a method of making such a polarizing glass article.